Starting a Dumas AVL. Many questions

[Kez]

I have had this kit for many years and decided to start building it. The quality of the door skin that Dumas used for the frames is really poor. They are only good for templates. Since I have two sheets of Occume marine ply, I have decided to build the entire boat using the marine ply. It is more flexible than birch but also lighter. I have always glassed the hull with 0.6 oz cloth so the flexibility should work well for sheeting the deck.

Still in the process of cutting the remaining pieces. But I am planning ahead before actually gluing the frames. The only two mods I am including are removing the recess in the strut area and not adding the ride pads to the sponsons. I have many questions:

1. I do not race but I do like to build it according to the scale hydro rules. Can I stick the tuned pipe out of the cowl? It will look ugly but if I do not add a muffler, it may be less obvious
2. If I run the exhaust out the transom like many of you do, it will weaken the transom. Will it be a problem?
3. Running the exhaust out the transom will also require removing the radio box and the doublers. The whole structure will now rely on the tunnel floor from flexing. Will it be a problem?
4. The ideal solution is to run the tuned pipe like Dumas showed in the plans but use a turbine cowling to hide the pipe. Which boat can this kit be built into that has a turbine cowling?
5. A blow over is inevitable. How do you mount the fins and the wing so that they will break away when it flips

I am sure I will have more questions as I go.

Thanks
k

 

[Mike Cathey]

I would recommend you go to Scott Meyers MP5 YouTube channel and most of your questions will be answered.

I will answer some questions for you.

1. If you ever do race the pipe/muffler being visible will not conform to the scale rules and most, if not all the model boat clubs have rules on the allowable Db levels and that would make you out of compliance, you need a muffler or a muffled pipe. Also, if you intend on running this boat on public water the racket w/o a muffler is bad for the hobby in general, not to mention your own hearing. You also need to keep in mind w/o the insurance of a sanctioning body you have no insurance in case you hit something or somebody with the boat. Your financial rear end is really on the line and could also cause the loss of the site to other model boaters. Make no mistake, these boats can do a lot of injury to someone or a boat. The one thing about racing is that people are willing to help a new guy and you can tap into a wealth of experience and help right at the race site. You might try to connect with the club in your area for help or just go to a race and see for yourself what the common building and installations are.

2. I can't tell from the photo what the thickness the transom is. 1/8" is adequate for an1/8th scale boat. You can add a doubler if you need to get there. I wouldn't worry too much about the bottom flexing. As you get the afterplane framed and sheeted and the deck goes on you will have all the strength needed. Most experienced builders will tell you that everything behind the sponsons is just along for the ride anyway. Most new builders tend to way overbuild and this adds weight and just kills performance. Putting the pipe inside the boat is not that big a deal and that is by far the most common way it's done. You just have to cut the frames behind the engine into a U-shape and leave the edges with a 1/4" of material. You can add some 3/16" by 3/16" Spruce as vertical gussets. You will need a 5 degree offset header for the motor to angle the pipe/muffler to the right of the strut.

3. Most 1/8th scale boats you see have a self contained radio box that is fastened to the bottom in front of the motor. The main fuel tank is usually under the right side deck. There's an article on the RC Unlimiteds website called "rigging a 8255 hull" that I wrote that has a step by step process of building a self-contained radio box (with photos). There's also a lot in the article about installing the hardware (motor mounts , strut, stuffing box and cable, mounting the rudder, ect.) and fuel systems.

4. If you are not planning to race in a IMPBA/NAMBA sanctioned club that has scale rules and are determined to have the pipe in the turbine tube, then you want to pick one of the earlier design turbine boats that the turbine tube extends almost to the transom. Newer design boats have much shorter tubes. Again I highly recommend you bury the pipe in the hull. Remember, if you want to follow the scale rules you will have to refer to, and build to, the Master Hull Roster for the correct dimensions of any particular boat. You can find the MHR on the RC Unlimiteds website.

Wesley's Custom Graphics in Marion IL. has a large number of graphics you can get for scale boats and Wes is a first class guy to do business with. Bill Brandt at Rattlesnake RC has most of the hardware parts and pieces you will need and he to is great to deal with and has a wealth of scale model knowledge to help you. Stu Barr of Crapshooter boats is the man for all things for CMB motors. You can usually find used motors here on IW.

5. I would again refer you to Scott's website on the wing deal. You can purchase fiberglass wing sets and mounting hardware from the RC Boat Co., they also have one piece fiberglass cowlings that is a lot easier and lighter than building one out of wood.

Hope this helps and welcome to model boating.

 

[Rich Jones]

Dumas Atlas Van Lines

Heres the link to a thread from back in maybe 2006 or so that I started under my old profile. There’s some good info in there if you want this boat to run right. Otherwise, it’s a taildragger and there’s not much you can do once it’s built that way. If you are going to take the time to cut out parts, make the recommended changes in the process. I never built the kit but I’m certain they will greatly improve the boat design. Regarding the wings, I’d install brackets, and then make or buy a fiberglass wing and upright set from RC Boat Company. Attach Them with nylon bolts so they break away in a blow over. Make sure they can float. I had an AVL that I finished as an Oberto turbine 1990, and the first time it went over, the uprights snapped in half.

 

[Kez]

Mike,

Thanks for your detailed reply. I watched Scott Meyers' videos this morning on building a gas 1/6 scale hydro and picked up some useful tips. I could not find your article 'rigging a 8255 hull' however. If you can forward a link that would be great.

On tuned pipes: Like you, I care about others and my own hearing and will do whatever I need to keep the noise level down. When I said not adding a muffler, I am thinking of trying an internal stinger. I also have a Magic Muffler that can be hidden under the cowl. In fact, I find the Prather muffler to be too loud. I am in the process of cutting one in half and then block off the internal passage forcing the flow from exiting straight out.

If I end up using a tuned pipe instead of the MM, I have a Picco muffled pipe and a Prather pipe to choose from.

I use aircraft birch for the transom. I will be laminating two 1/8 ply to make a 1/4" piece.

For the tuned pipe header, I have two headers meant for F3A pattern planes. I think it may work better than a 5 degree marine header. If it does not work, I can always cut the header and weld it at any angle I want.

Thanks, k

 

[Kez]

Dumas Atlas Van Lines

Heres the link to a thread from back in maybe 2006 or so that I started under my old profile. There’s some good info in there if you want this boat to run right. Otherwise, it’s a taildragger and there’s not much you can do once it’s built that way. If you are going to take the time to cut out parts, make the recommended changes in the process. I never built the kit but I’m certain they will greatly improve the boat design. Regarding the wings, I’d install brackets, and then make or buy a fiberglass wing and upright set from RC Boat Company. Attach Them with nylon bolts so they break away in a blow over. Make sure they can float. I had an AVL that I finished as an Oberto turbine 1990, and the first time it went over, the uprights snapped in half.

Hi Rich,

Thanks for the link to the thread. Lots of good information there. Looks like I am on the right track by eliminating the strut recess and not adding the sponson ride pads. In fact, I lowered the sponson by another 1/8" and will be adding a 1/16" strip on the inside surface.

If you look at the kit, there really isn't a lot of area where I can cut lightening holes aft of the sponson. One mistake I made was to cut out the opening from the two cockpit side. If I decided not to build the radio compartment like the Dumas kit, I will have to cover them up.

I am thinking of building a long and narrow radio box to the left side thus allowing me to run the pipe on the right to exit the transom. I prefer not to put the fuel tank under the deck because I will have to cut an access opening in the tunnel floor.

Thanks,
k

 

[Hydro Junkie]

Kez, if you're going to use a laminated transom, I would suggest you cut lightening holes in the inner piece in the areas under the deck. As already stated, weight aft of the sponsons needs to be kept to a minimum and that's one place you can take weight out of the hull. With that being said, you can cut lightening holes out of the almost all of the frames in several locations throughout the length of the hull as shown in the pictures below:
This build was from a Dumas Pak kit that I made several changes to. I used all birch plywood, widened the tunnel, reduced the depth of the sponsons and changed the bottom profile by moving the bottom break back to frame 6 from frame 4. By the time I was done, the boat looked like this:

When I built my first boat, way back in 1982, it was the same kit. I built it per the instructions and ended up with a barge that was underpowered and massively overweight at over 8lbs. Even with all of the changes I made, it came out at just over 5lbs as shown

 

[Mike Cathey]

Mike,

Thanks for your detailed reply. I watched Scott Meyers' videos this morning on building a gas 1/6 scale hydro and picked up some useful tips. I could not find your article 'rigging a 8255 hull' however. If you can forward a link that would be great.

On tuned pipes: Like you, I care about others and my own hearing and will do whatever I need to keep the noise level down. When I said not adding a muffler, I am thinking of trying an internal stinger. I also have a Magic Muffler that can be hidden under the cowl. In fact, I find the Prather muffler to be too loud. I am in the process of cutting one in half and then block off the internal passage forcing the flow from exiting straight out.

If I end up using a tuned pipe instead of the MM, I have a Picco muffled pipe and a Prather pipe to choose from.

I use aircraft birch for the transom. I will be laminating two 1/8 ply to make a 1/4" piece.

For the tuned pipe header, I have two headers meant for F3A pattern planes. I think it may work better than a 5 degree marine header. If it does not work, I can always cut the header and weld it at any angle I want.

Thanks, k

Go to the RCU website Home page, click on Resources, then click on Special Articles and you will find it.

 

[Kez]

Mark,
The Pak looked good. I see you replaced the door skin as well. In fact, I still have this kit in my collection. In the background of the attached photo is a 21 size Drag'n Fli. I have built all three sizes over the last 40 years. This one in the picture is the 4th 20 size that I have built. Finished it last year but have yet to get it wet. I was able to keep the weight to 4.5lb.

Back to this AVL, I copied the cockpit sides from the original kit and cut the two openings. If I did not cut these two openings, I can use a hole saw to cut lightening holes in the frames like you did to the Pak. But unless I seal these two openings, I cannot do that or I have to really seal the inside very well.

I am planning on cutting lightening holes in the sponson formers. But that will not help with the CG too much. I know I will need to add weight to the nose.

The Occume ply is lighter than birch so I hope it will not need to add too much weight to put the CG where I want it.

Thanks, k

 

[Rich Jones]

Hi Rich,

Thanks for the link to the thread. Lots of good information there. Looks like I am on the right track by eliminating the strut recess and not adding the sponson ride pads. In fact, I lowered the sponson by another 1/8" and will be adding a 1/16" strip on the inside surface.

If you look at the kit, there really isn't a lot of area where I can cut lightening holes aft of the sponson. One mistake I made was to cut out the opening from the two cockpit side. If I decided not to build the radio compartment like the Dumas kit, I will have to cover them up.

I am thinking of building a long and narrow radio box to the left side thus allowing me to run the pipe on the right to exit the transom. I prefer not to put the fuel tank under the deck because I will have to cut an access opening in the tunnel floor.

Thanks,
k

I did this on my Phil Thomas T-5 hull. Phil has a 2 x 7 radio box that may work for you.

 

[Hydro Junkie]

I missed this part earlier and you have me curious, why did you lower the sponson? I'm doing the exact opposite on a scale 1973 Pay'N Pak and on a gas scale 1978 Madison. I raised the sponsons to scale depth and am adding ride pads to get to the boat to ride at the proper height. In my case, however, I have the access to the full-sized boat so making the changes to Newton plans is much easier. One thing to remember is that the less area in contact with the water means that much less drag and that much more speed, BUT ONLY TO A POINT!!!

The bottom of the sponson is 1/4" mahogany covered with FG and the ride pad is laminated wood strips with ply/FG over it in the front and beveled areas with a metal running surface at the rear. From what I've seen, many of the boats of that era were built the same way.

 

[Kez]

'Lowering the sponson' was incorrect. I was thinking in term of the boat sitting upside down on my work bench ;-)
I actually meant lowering the tunnel floor by removing 1/8" from the sponson inside while keeping the same AOA and dihedral. I will then glue a strip of 1/16" ply in a similar fashion as the full size boat's sponsons. If I built the boat according to the kit, the sponson will be too deep.

 

[Hydro Junkie]

Okay, that sounds better. Both Dumas boats and the Newton 108 plans have the sponsons extremely deep. That's why I shaved 3/8" off the bottom of the Dumas Pak sponson insides and reworked the air traps into something closer to scale. I also removed a bunch of material off of my scale Pak sponson insides as well as reworking the air traps to make them scale.